A fast identification method for phase-to-phase short circuit based on wavelet packet transform

Abstract

The present invention discloses a method for rapid identification of interphase short circuits based on wavelet packet transform, comprising the following steps: obtaining attribute information of a line fault point; obtaining phase current information at the protection installation locations at both ends of the fault point, and performing phase-mode transformation to obtain mode current information; calculating differential current using the mode current information; decomposing and reconstructing the differential current using db4 wavelet to obtain several high-frequency bands and one low-frequency band; calculating the high-frequency energy proportion of the high-frequency band and the low-frequency energy proportion of the low-frequency band in each data window and drawing an energy proportion curve; and determining the fault type based on the amplitude and duration of the energy proportion. The effects achieved by this solution are: rapid identification after an interphase short circuit fault occurs, unaffected by the system operating mode and power frequency oscillation, and no dead zone within the area, unaffected by the fault location, thereby improving protection reliability.

Description

Technical Field

[0001] The present invention relates to the technical field of line fault identification, and in particular to a method for quickly identifying phase-to-phase short circuits based on wavelet packet transform. Background Art

[0002] Relay protection is the first line of defense for ensuring the safe operation of power grids. In practical engineering applications, relay protection that responds to power-frequency electrical quantities dominates. With the widespread use of power electronics, power system faults have become more complex, and power-frequency variable protection is facing the problem of not being able to operate correctly.

[0003] The above information disclosed in this Background section is only for enhancement of understanding of the background of the application and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of incorrect action of protective switches due to inaccurate identification of phase-to-phase short-circuit faults, and proposes a method for rapid identification of phase-to-phase short circuits based on wavelet packet transform. This application utilizes the characteristics that after a fault occurs, the high-frequency component of the fault current will be rapidly generated and rapidly decayed, and it is not affected by the system operation mode, power frequency oscillation, etc., to constitute transient quantity protection, which can identify phase-to-phase short-circuit faults (excluding single-phase grounding faults) more quickly and accurately.

[0005] A technical solution provided in an embodiment of the present invention is a method for quickly identifying phase short circuits based on wavelet packet transform, comprising the following steps:

[0006] Obtain attribute information of line fault points;

[0007] Obtaining phase current information at protection installations at both ends of the fault point, and performing phase-mode transformation to obtain mode current information;

[0008] Calculating a differential current using the mode current information;

[0009] Decomposing and reconstructing the differential current by using db4 wavelet to obtain several high-frequency bands and one low-frequency band;

[0010] Calculate the high-frequency energy ratio of the high-frequency band and the low-frequency energy ratio of the low-frequency band in each data window and draw an energy ratio curve;

[0011] The fault type is determined based on the magnitude and duration of the energy ratio.

[0012] Preferably, the attribute information includes connection topology information of the line fault point, and the connection topology information includes a first relay protection device and a second relay protection device respectively arranged at protection installation locations at both ends of the fault point.

[0013] As a preference, the phase mode transformation obtains the mode current information, and the formula is as follows:

[0014]

[0015] in, is the modulus current of the first relay protection device; is the modulus current of the first relay protection device; is the phase a mode current at the first relay protection device end; is the b-phase mode current at the first relay protection device end; is the phase C mode current at the first relay protection device end; is the phase a mode current at the second relay protection device end; is the b-phase mode current at the second relay protection device end; is the C-phase mode current at the second relay protection device end.

[0016] Preferably, the differential current is calculated using the mode current information, and the formula is as follows:

[0017]

[0018] in, is the differential current.

[0019] Preferably, the decomposition and reconstruction of the differential current by using the db4 wavelet to obtain a plurality of high-frequency bands and a low-frequency band comprises the following steps:

[0020] The db4 wavelet is used to perform 7-layer wavelet packet decomposition and reconstruction on the differential current, and the differential current waveform is decomposed into 7 high-frequency bands and 1 low-frequency band.

[0021] Preferably, the seven high frequency bands include d1: 50-100kHz; d2: 25-50kHz; d3: 12.5-25kHz; d4: 6.25-12.5kHz; d5: 3.125-6.25kHz; d6: 1.5625-3.125kHz; d7: 781.25-1562.5Hz;

[0022] The 1 low-frequency band is a7: 0-781.25 Hz.

[0023] Preferably, the step of calculating the high-frequency energy ratio of the high-frequency band and the low-frequency energy ratio of the low-frequency band in each data window and drawing an energy ratio curve comprises:

[0024] The accumulated energy of seven high-frequency bands is used as the high-frequency energy; the energy of one low-frequency band is used as the low-frequency energy;

[0025] The sum of the energy accumulation value and the low-frequency energy is taken as the total frequency energy in the data window.

[0026] Preferably, the determining of the fault type according to the amplitude and duration of the energy proportion includes:

[0027] When the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is 0.2 and less than 0.7, and drops below 0.2 within time t, it is determined to be an internal fault;

[0028] If the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is greater than 0.7 and the duration is greater than t, it is determined to be an out-of-zone fault.

[0029] Beneficial effects of the present invention: This application utilizes the characteristics that after a fault occurs, the high-frequency component of the fault current will be rapidly generated and rapidly attenuated, and it is not affected by the system operation mode, power frequency oscillation, etc., to constitute transient quantity protection. Phase-to-phase short-circuit faults (excluding single-phase grounding faults) can be identified more quickly and accurately. Through this technical solution, after a phase-to-phase short-circuit fault occurs, it can be quickly identified, and is not affected by the system operation mode and power frequency oscillation, etc., and there is no dead zone in the area, and it is not affected by the fault location, thereby improving the protection reliability.

[0030] The above content of the invention is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other features, objects, and advantages of the present invention will become more apparent upon reading the detailed description of the non-limiting embodiments made with reference to the following drawings. The drawings are for the purpose of illustrating preferred embodiments only and are not to be construed as limiting the present invention. Like reference characters are used throughout the drawings to designate like parts.

[0032] Figure 1 The present invention is a flowchart of a method for quickly identifying phase short circuits based on wavelet packet transform.

[0033] Figure 2 Schematic diagram of line fault of the present invention. DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific implementation method described herein is only an optimal embodiment of the present invention, which is only used to explain the present invention and does not limit the scope of protection of the present invention. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0035] Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flow charts. Although the flow charts describe the operations (or steps) as sequential processes, many of the operations (or steps) therein can be performed in parallel, concurrently, or simultaneously. In addition, the order of the operations can be rearranged. The process can be terminated when its operations are completed, but can also have additional steps not included in the figures; the process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0036] Example: Figure 1 As shown, a method for quickly identifying phase-to-phase short circuits based on wavelet packet transform includes the following steps: S1, obtaining attribute information of a line fault point.

[0037] Specifically, the attribute information includes connection topology information of the line fault point, and the connection topology information includes a first relay protection device and a second relay protection device respectively provided at protection installation locations at both ends of the fault point.

[0038] S2. Acquire phase current information at protection installations at both ends of the fault point, and perform phase-mode conversion to obtain mode current information.

[0039] Phase-mode transformation obtains mode current information, and the formula is as follows:

[0040]

[0041] in, is the modulus current of the first relay protection device; is the modulus current of the first relay protection device; is the phase a mode current at the first relay protection device end; is the b-phase mode current at the first relay protection device end; is the phase C mode current at the first relay protection device end; is the phase-a mode current at the second relay protection device end; is the b-phase mode current at the second relay protection device end; is the C-phase mode current at the second relay protection device end.

[0042] S3. Calculate the differential current using the analog current information.

[0043] The differential current calculation formula is as follows:

[0044]

[0045] in, is the differential current.

[0046] S4. Decompose and reconstruct the differential current using db4 wavelet to obtain several high-frequency bands and one low-frequency band.

[0047] Specifically, the db4 wavelet is used to perform 7-layer wavelet packet decomposition and reconstruction on the differential current, and the differential current waveform is decomposed into 7 high-frequency bands and 1 low-frequency band.

[0048] Furthermore, the seven high-frequency bands include d1: 50-100kHz; d2: 25-50kHz; d3: 12.5-25kHz; d4: 6.25-12.5kHz; d5: 3.125-6.25kHz; d6: 1.5625-3.125kHz; d7: 781.25-1562.5Hz; and one low-frequency band is a7: 0-781.25Hz.

[0049] S5. Calculate the high-frequency energy ratio of the high-frequency band and the low-frequency energy ratio of the low-frequency band in each data window and draw an energy ratio curve.

[0050] Specifically, the accumulated energy value of seven high-frequency bands is used as the high-frequency energy; the energy of one low-frequency band is used as the low-frequency energy;

[0051] The sum of the energy accumulation value and the low-frequency energy is taken as the total frequency energy in the data window.

[0052] S6. Determine the fault type according to the magnitude and duration of the energy proportion.

[0053] Specifically, when the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is 0.2 and less than 0.7, and drops below 0.2 within time t, it is determined to be an internal fault;

[0054] If the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is greater than 0.7 and the duration is greater than t, it is determined to be an out-of-zone fault.

[0055] In order to enable those skilled in the art to have a more complete understanding and grasp of the technical concept of this application, this application lists the following specific embodiments to further illustrate the scheme. The specific examples serve to further illustrate this application and cannot be used to limit the specific implementation scope of the invention.

[0056] like Figure 2 As shown in the figure, assuming that a non-single-phase grounding short circuit fault occurs at a certain line f1 in the system, the phase mode transformation of the three-phase current at the protection M and N is performed to obtain the differential current. The differential current is reconstructed after a 7-layer wavelet packet decomposition using the db4 wavelet. The original differential current waveform can be decomposed into 7 high-frequency bands d1 to d7 and 1 low-frequency band a7.

[0057] To minimize the transmission impact of the line and highlight the role of the line boundary, the observation frequency band is selected to be below 100 kHz, so the sampling frequency is selected to be 200 kHz. Therefore, the resulting eight frequency bands, from high to low, are 50-100 kHz, 25-50 kHz, 12.5-25 kHz, 6.25-12.5 kHz, 3.125-6.25 kHz, 1.5625-3.125 kHz, 781.25-1562.5 Hz, and 0-781.25 Hz. Band a7 (0-781.25 Hz) is defined as the low-frequency band, and bands d1 to d7 are defined as the high-frequency bands.

[0058] The sampling point data of each frequency band is squared to calculate the energy value of each frequency band.

[0059] For a sampling frequency of 200 kHz, there is a sampling point every 5 μs. The length of the data window is set to be 200 sampling points, that is, 1 ms. For each frequency band, the total energy in the data window is calculated.

[0060] Calculate the ratio of the high- and low-frequency waveform energies within each data window: the energy in band a7 is the low-frequency energy, and the sum of the energies in bands d1 through d7 is the high-frequency energy. Obviously, the sum of the high- and low-frequency energy is the total energy, and a ratio can be calculated for each data window.

[0061] The data window is kept sliding at a length of 1ms, and the energy proportions of the high-frequency and low-frequency bands are plotted.

[0062] Since there is only a 50Hz power frequency component and no high-frequency component during normal operation, when the line is operating normally, the energy proportion curve of the high-frequency band is zero.

[0063] When a phase-to-phase short circuit occurs within the zone, the high-frequency component rapidly generates and decays, resulting in a narrow "spike" in the high-frequency energy waveform. Because the 50Hz differential current component is high during a zone fault and rapidly increases after the fault, the height of the high-frequency spike is approximately 0.5.

[0064] When a phase-to-phase short circuit fault occurs outside the zone, since the differential current content of the 50Hz component is very small, only the high-frequency component is generated rapidly when the fault just occurs, so a wider "peak" will be generated, and its amplitude can reach close to 1.

[0065] Calculate the proportion of high-frequency waveform energy within a 1ms data window and plot a high-frequency energy proportion curve. Under normal circumstances, the curve amplitude is 0. When the amplitude of the high-frequency energy proportion curve is greater than 0.2, a fault has occurred. If the amplitude of the high-frequency energy proportion curve is greater than 0.2 and less than 0.7, and the amplitude of the high-frequency energy proportion curve drops below 0.2 within 2.5ms, it is determined to be an in-zone fault. If the amplitude of the high-frequency energy proportion curve is greater than 0.7 and the duration is greater than 2.5ms, it is determined to be an out-of-zone fault. Similarly, the analysis of the low-frequency energy proportion curve is the same as that of the high-frequency energy proportion curve and is not repeated here.

[0066] The specific implementation described above is a preferred implementation of the method for rapid identification of phase short circuits based on wavelet packet transform of the present invention, and is not intended to limit the specific implementation scope of the present invention. The scope of the present invention includes but is not limited to this specific implementation. All equivalent changes made in accordance with the shape and structure of the present invention are within the scope of protection of the present invention.

Claims

1. A method for rapid identification of phase short circuit based on wavelet packet transform, characterized by: The steps include: Obtain attribute information of line fault points; Obtaining phase current information at protection installations at both ends of the fault point, and performing phase-mode transformation to obtain mode current information; Calculating a differential current using the mode current information; Decomposing and reconstructing the differential current by using db4 wavelet to obtain several high-frequency bands and one low-frequency band; Calculate the high-frequency energy ratio of the high-frequency band and the low-frequency energy ratio of the low-frequency band in each data window and draw an energy ratio curve; The fault type is determined based on the magnitude and duration of the energy ratio.

2. The method for quickly identifying interphase short circuits based on wavelet packet transform according to claim 1, characterized in that: The steps include: The attribute information includes connection topology information of a line fault point, and the connection topology information includes a first relay protection device and a second relay protection device respectively provided at protection installation locations at both ends of the fault point.

3. The method for quickly identifying interphase short circuits based on wavelet packet transform according to claim 2, characterized in that: The steps include: Phase-mode transformation obtains mode current information, and the formula is as follows: in, is the modulus current of the first relay protection device; is the modulus current of the first relay protection device; is the phase a mode current at the first relay protection device end; is the b-phase mode current at the first relay protection device end; is the phase C mode current at the first relay protection device end; is the phase-a mode current at the second relay protection device end; is the b-phase mode current at the second relay protection device end; is the C-phase mode current at the second relay protection device end.

4. The method for quickly identifying interphase short circuits based on wavelet packet transform according to claim 3, characterized in that: The steps include: The differential current is calculated using the mode current information, and the formula is as follows: in, is the differential current.

5. The method for rapid identification of interphase short circuits based on wavelet packet transform according to claim 1, characterized in that: The steps include: Decomposing and reconstructing the differential current by using the db4 wavelet to obtain a plurality of high-frequency bands and a low-frequency band includes the following steps: The db4 wavelet is used to perform 7-layer wavelet packet decomposition and reconstruction on the differential current, and the differential current waveform is decomposed into 7 high-frequency bands and 1 low-frequency band.

6. The method for quickly identifying interphase short circuits based on wavelet packet transform according to claim 5, characterized in that: The steps include: The seven high frequency bands include d1: 50-100kHz; d2: 25-50kHz; d3: 12.5-25kHz; d4: 6.25-12.5kHz; d5: 3.125-6.25kHz; d6: 1.5625-3.125kHz; d7: 781.25-1562.5Hz; The 1 low-frequency band is a7: 0-781.25 Hz.

7. The method for quickly identifying interphase short circuits based on wavelet packet transform according to claim 5, characterized in that: The steps include: The step of calculating the high-frequency energy ratio of the high-frequency band and the low-frequency energy ratio of the low-frequency band in each data window and drawing an energy ratio curve comprises: The accumulated energy of seven high-frequency bands is used as the high-frequency energy; the energy of one low-frequency band is used as the low-frequency energy; The sum of the energy accumulation value and the low-frequency energy is taken as the total frequency energy in the data window.

8. A method for rapid identification of interphase short circuits based on wavelet packet transform according to claim 1 or 7, characterized in that: The steps include: The determining of the fault type according to the magnitude and duration of the energy proportion includes: When the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is 0.2 and less than 0.7, and drops below 0.2 within time t, it is determined to be an internal fault; If the amplitude of the high-frequency energy ratio or the amplitude of the low-frequency energy ratio is greater than 0.7 and the duration is greater than t, it is determined to be an out-of-zone fault.

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